1. Field of the Invention
The present invention relates to building heating and air conditioning thermostats of the type usually mounted on a wall to control the ambient air temperature.
2. Description of the Prior Art
Historically, building heating and air conditioning control has been performed by wall mounted thermostats. Such thermostats can be readily read to ascertain the actual temperature and a sliding scale or rotary dial is manipulated to alter the desired temperature. A temperature sensing element closes electrical contacts to turn on the heating or air conditioning unit when a predetermined temperature differential exists between actual temperature and desired temperature. The furnace or air conditioner is cut off by opening the contacts once actual temperature achieves the desired temperature to within a predetermined smaller tolerable limit.
With increasing concerns for energy conservation brought about by a progressive diminution of energy resources, more sophisticated temperature control systems have been made. For example, U.S. Pat. No. 3,948,441 describes a wall mounted thermostat in which provision is afforded to set time variable thermostat settings such as a lower night temperature and a higher daytime temperature as the desired temperatures. A clock is provided so that the target or desired temperature is automatically adjusted to the appropriate setting depending upon the time of day. The system described in the aforesaid patent provides for the timer or clock mechanism to be isolated from the line voltage and driven only from a rechargeable battery source. The battery source itself is charged at a very low rate by 24 volt 60 cycle current during the time the thermostat is off. This arrangement permits a thermostat having only two wires for control purposes to be retrofitted with a time variable energy saving thermostat that uses the same two wires for two purposes i.e. battery charging and thermostatic controlling of the heating system. Generally, because conventional wall thermostats use coiled bimetal strips for opening and closing electrical contacts to relays or contactors associated with the heating and air conditioning devices, sensitivity and thermal inertia are limiting factors to efforts to achieve better control.
In another current thermostat employing a digital clock, temperature sensing is provided by an epoxy-coated thermistor operated in a linear voltage versus temperature circuit. Comparison with reference settings in electronic amplifiers causes the appropriate heating or cooling element to be energized as conditions demand. Timing for relative temperature shifts between two alternative temperature levels is derived from a 60 hertz low voltage line obtained from an existing low voltage supply line. The return path is through an inactive heating/cooling relay element, or through an active relay element via a transformer in the case of a heat-only system.